In a wireless communication system, a transmitter employs power amplifiers (PA) to boost signal power for radio transmission. To be efficient in occupying frequency spectrum, modern communication signals have a large peak-to-average power ratio (PAR). The capability to reach peak power and to maintain energy efficiency at the average power levels is contradictory for conventional single branch PAs. More sophisticated PA architectures employ multiple PA branches to satisfy both the power and the efficiency requirements. Among these PA architectures, the most widely used in wireless infrastructures is Doherty Power Amplifiers (DPA) described in e.g., Andrei Grebennikov and Senad Bulja, “High-Efficiency Doherty Power Amplifiers: Historical Aspect and Modern Trends”, Proceedings of the IEEE, Vol. 100, No. 12, December 2012.
A DPA comprises of at least 2 PA branches, namely a main and an auxiliary branch. In operation, an auxiliary PA in the auxiliary branch injects power into a main PA in the main branch to modulate its effective load impedance according to a specific pattern, hereafter referred to as Doherty load modulation, such that total efficiency is maximized not only at the peak-power but also at the average-power level. At any other power levels between the peak-power and the average-power level, the efficiency degrades unavoidably. The degradation is more severe for larger PAR signals. A common remedy to enhance efficiency at medium power levels is to employ more PA branches.
A 3-way DPA comprises of 3 PA branches, namely a main PA and two auxiliary PAs, Aux1 and Aux2. Aux1 modulates the load of the main PA the same as that in a 2-way DPA, to maximize efficiency at the low and medium power levels. Aux2 modulates the loads of both the main and Aux1 PAs to maximize efficiency at the peak power level. As a result, total efficiency at 3 different power levels can be maximized. The energy saving is considerable in high PAR applications.
The power from multiple PA branches needs to be combined properly for Doherty load modulation. The existing power combiners for 3-way DPAs have problems in size, bandwidth, complexity and cost.
Conventional Doherty power combiners employ multiple pieces of quarter-wavelength transmission lines as impedance inverters. The size of these transmission lines are determined by operating frequencies and the dielectric materials and thus is difficult to miniaturize. Their effective bandwidth is also limited.
An alternative technique disclosed in US2004189380 solves both the size and bandwidth problems for a 2-way DPA by employing a high reflection terminated hybrid coupler as a 2-way Doherty power combiner. An extension scheme for more than 2 branches of PAs is also published. However, an addition of another branch to make it a 3-way DPA requires the addition of another hybrid coupler with possibly a different coupling factor, duplicating the size, complexity and cost.
In WO2014075735, a 3-way DPAs employing a coupler circuit as the 3-way Doherty power combiner is disclosed. However, the coupler circuit is a two-section coupling structure comprising feeding ports for the amplifier stages of the main amplifier and each of the auxiliary amplifiers. Thus size, complexity or cost are not decreased with this 3-way DPA.
The aforementioned size and cost drawbacks are especially acute for implementations in Monolithic Microwave Integrated Circuits (MMIC), where a majority of chip area of the MMIC is occupied by passive components. In these implementations, an additional coupler together with associated interconnections could increase the chip area and cost by more than 50%.